1. Field of the invention
The present invention relates to an electron gun for high brightness in which a lanthanum hexaboride (LaB.sub.6) single crystal is used.
2. Description of the Prior Art
The thermionic electron emission cathode made of a LaB.sub.6 single crystal (hereinafter referring to as a LaB.sub.6 single crystal cathode) exhibits high brightness, long life and excellent electron beam stability and accordingly, it is practically used as a cathode of an electron gun for electron beam applied instruments such as a scanning type electron microscope and an electron beam exposure instrument.
The LaB.sub.6 single crystal cathode has a higher purity of the cathode source in comparison with the sintered LaB.sub.6 cathode. Moreover, it has a smooth surface whereby the stability of the electron beam is especially superior to be able to limit variations of the electron beam to 1%/hr. or less. However, prior efforts have not succeeded in developing an electron gun having a LaB.sub.6 single crystal cathode which maintains high brightness for a long time. Thus, there is a need to obtain a LaB.sub.6 single crystal cathode having excellent brightness and long life as well as a high-stability electron beam.
LaB.sub.6 single crystal cathodes having each &lt;100&gt;, &lt;110&gt; or &lt;111&gt; direction as the axial direction of the cathode have been respectively studied and brightnesses thereof have been measured by many researchers to satisfy the requirement as disclosed in F. J. Hohn, T. H. P. Chang, A. N. Broers, "Comparative measurements on the High Brightness LaB.sub.6 Electron Gun using Sintered and Single Crystal Cathodes Materials", Proc. Microcircuit Engineering, 79 Aachen 31, and Takigawa, Yoshii, Sasaki, Motoyama and Meguro, "Emission Characteristics for &lt;100&gt; &lt;110&gt; and &lt;111&gt; LaB.sub.6 Cathodes", Japanese Journal of Applied Physics. 19(9) 557-540 1980.
In the prior art, it is reported that the brightness of the LaB.sub.6 single crystal cathode having a &lt;100&gt; direction is the highest and the brightness of the LaB.sub.6 single crystal cathode having a &lt;110&gt; direction is inferior and that of a &lt;111&gt; direction is further inferior.
The inventors have determined that the electron gun having a LaB.sub.6 single crystal cathode having a &lt;100&gt; direction imparts brightness by 10 times or more of the brightness of tungsten (W) hair pin cathode at the initiation, however, the brightness decreases over time to decrease to about 2 times of that of the tungsten hair pin cathode.
On the other hand, when a single crystal cathode having a &lt;110&gt; direction or a &lt;111&gt; direction is used in an electron gun in the combination of the flat (conventional) Wehnelt shown in FIG. 1, a satisfactory brightness could not be given from the initiation as shown in FIG. 2.